Article ID: 183229 - Last Review: October 31, 2006 - Revision: 1.1

RAS Uses Fixed TCP/IP MTU Size

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This article was previously published under Q183229

IMPORTANT: This article contains information about modifying the registry. Before you modify the registry, make sure to back it up and make sure that you understand how to restore the registry if a problem occurs. For information about how to back up, restore, and edit the registry, click the following article number to view the article in the Microsoft Knowledge Base:
256986  (http://support.microsoft.com/kb/256986/EN-US/ ) Description of the Microsoft Windows Registry
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SYMPTOMS

When you run a program over a Remote Access Service (RAS) connection, you may encounter one of the following symptoms:
  • You are able to download data normally but you are not always able to upload data successfully.
  • Your Internet mail or news reading programs may time out or stop responding (hang) when you attempt to send new messages.
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CAUSE

This behavior can occur because Windows NT RAS uses a fixed size Maximum Transmission Unit (MTU) of 1500.
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RESOLUTION

WARNING: If you use Registry Editor incorrectly, you may cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that you can solve problems that result from using Registry Editor incorrectly. Use Registry Editor at your own risk.

To resolve this problem, obtain the latest service pack for Windows NT version 4.0. For additional information, click the article number below to view the article in the Microsoft Knowledge Base:
152734  (http://support.microsoft.com/kb/152734/EN-US/ ) How to Obtain the Latest Windows NT 4.0 Service Pack
NOTE: Your computer must run Windows NT 4.0 Service Pack 3 before you apply this fix. This hotfix can also be applied to the Routing and Remote Access Service Upgrade.

This hotfix also includes the fix referred to in the following article in the Microsoft Knowledge Base:
163251  (http://support.microsoft.com/kb/163251/EN-US/ ) STOP 0xA Due to Buffer Overflow in Ndiswan.sys
There are two new registry keys to address this issue, one that allows you to edit the MTU for a RAS connection and another that allows you to edit the MTU for a Point-to-Point Tunneling Protocol (PPTP) connection. To edit the registry:
  1. Start Registry Editor (Regedt32.exe or Regedit.exe) and locate the following registry key:
    HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\NdisWan\Parameters
  2. Create the following two new data values using the following information:
    Value name: IPMTU
    Data type : REG_DWORD
    Data range: 1 - 1500 (default is 1500)

    Value name: TunnelMTU
    Data type : REG_DWORD
    Data range: 1 - 1500 (default is 1400)
    NOTE: Setting these values to the smallest MTU of your data path will prevent fragmentation.
  3. Restart the computer.
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STATUS

Microsoft has confirmed this to be a problem in Windows NT version 4.0. This problem was first corrected in Windows NT 4.0 Service Pack 4.
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MORE INFORMATION

Using the EnablePMTUDiscovery and/or EnablePMTUBHDetect registry values will have no effect because the TCP/IP MTU for a RAS connection is a fixed value.

For additional information on TCP/IP, please see the following white paper available on the Microsoft ftp server:
File Name: Tcpipimp2.doc
Location: ftp://ftp.microsoft.com/bussys/winnt/winnt-docs/papers/ (ftp://ftp.microsoft.com/bussys/winnt/winnt-docs/papers/)
Title: "Microsoft Windows NT 3.5/3.51/4.0: TCP/IP Implementation Details TCP/IP Protocol Stack and Services, Version 2.0"
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APPLIES TO
  • Microsoft Windows NT Server 4.0 Standard Edition
  • Microsoft Windows NT Workstation 4.0 Developer Edition
  • Microsoft Windows NT Server 4.0 Enterprise Edition
  • Microsoft Routing and Remote Access Service Update for Windows NT Server 4.0
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Keywords: 
kbbug kbfix KB183229
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